A problem is that the current deployment of Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) in North America, Europe and elsewhere is primarily limited to routes where optical fibers exist between communicating SONET/SDH Network Elements. Due to the cost of installing a fiber optic infrastructure, these SONET/SDH routes are found primarily between telephone central offices where the cost of deploying the SONET/SDH optic infrastructure results in the greatest benefit. The result of this economic reality is that there is a large number of electrical transmission links making up the telephony access infrastructure, which could benefit from the additional capabilities provided by the SONET/SDH format, but for which the cost of replacing the existing electrical transmission links with optic fiber cannot be cost justified.
A proposed standard method for electrically interconnecting SONET equipment at the VT 1.5 level of the SONET hierarchy has been suggested by A. Turudic in the ANSI T1 Committee Contribution Document Number T1X1.5/93-008 dated Mar. 4, 1993. A mechanism is there proposed for interconnecting SONET equipment islands with a copper based network using electrical VT 1.5 for transport using a CEPT PCM-30 format as the electrical interface. A second proposal, using a 1.728 Mb/s electrical interface is shown in Committee T1 Contribution Document Number T1X1.5/94/068 dated Apr. 25, 1994. That proposal specifies a nominal line rate of 1.728 Mb/s using bipolar 8-zero substitution (B8ZS) on a balanced twisted pair. Essentially, the DS-1 bit rate is increased significantly to accommodate the VT 1.5 bandwidth.
However, these proposals are at the sacrifice of electrical compatibility with the existing DS-1 infrastructure for increased payload and as yet there has been no format agreed upon for transporting individual virtual tributaries or tributary units, particularly of size VT-1.5 and of size VT-2/TU-12.